A variety of seed corn dryers currently exist. The two main dryer categories are single pass and dual path dryers. Seed corn is typically dried on the cob and shelled after drying. Single pass dryers use air for a single pass through the seed corn bed to be dried. The air may be heated before passing through the bed and is then typically dumped to the outside without further use. As the incoming air passes through the seed bed, the seeds nearest the air inlet lose moisture and the air gains moisture. A gradient of seed moisture is established across the depth cf the seed bed. The used, moist air has less drying power than the incoming, dry air. When seed corn is especially dry or the air is especially humid, the moisture gradient across the bed depth may move through the bed at especially slow speeds or even remain stationary, as moist air is unable to dry moist corn within a reasonable time.
Reversible single pass dryers remedy this problem by allowing the air to be directed at either the bed top or bottom. Air may be directed through the seed bed from bottom to top, substantially drying the seed bed bottom half. The air is then directed through the seed bed from top to bottom, substantially drying the seed bed top half. The need to reverse the air flow through the seed corn bed is based in part on the relative humidity of the drying air, the initial moisture content of the seed corn, the desired final moisture of the seed corn, and drying parameters set by customers.
Dual path seed corn dryers use the drying air to dry two seed corn beds. The dry air first passes through a bed of drier seed corn that has likely been in the dryer for a long time period. The air is then routed through a second bed of seed corn that is wetter, likely having been in the dryer for a shorter time period. After the drier seed corn reaches target moisture, it can be removed from the bin and replaced by new, moist seed corn from the field. In the meantime, the initial, wetter seed corn has become drier. The air can now be routed first through the now drier seed corn and then through the wet seed corn. The air is thus routed through dual paths, including a first path through a first bin containing dry corn and a second bin containing moist corn.
Seed corn dryers operate within a narrow window of time. Seed corn cannot be dried until mature and viable kernels have been produced. Even at that time, further drying in place is normally desirable. The earliest seed corn can normally be dried is the beginning of September. Seed corn cannot be dried for use as seed corn if the kernels have been damaged by frost so as to impair the viability of the seed. The end of the drying period normally occurs about the beginning of October. The seed corn drying window is thus nominally one month long, with the seed corn dryers lying unused for the remainder of the year. In the period of use, a limited number of batches or "turns" can be dried. In a conventional dual path dryer, only about six batches of corn can be dried.
Both single pass seed corn dryers and dual path seed corn dryers have desirable attributes, and both types continue to be built. Single pass dryers allow rapid drying of seed corn and more batches or turns of corn through the dryer in a given time period. Single pass dryers allow drying of corn having a high moisture content. Single pass dryers are not particularly energy efficient, dumping moderately humid air after only a single pass instead of using the remaining drying potential of the air. Dual path dryers are more fuel efficient as the heated drying air is used twice, once in each pass through the corn.
Many dual path corn dryers were built for reasons of energy efficiency out of concern for gas costs and availability. The dual path dryers can be adequate in seasons having only moderately moist corn and a long period in which to bring in the corn. In particular, dual path dryers are more suitable where there is no threat of cold weather forcing rapid harvesting of the corn. Extended mild weather allows the corn to dry in place and does not force the dryer operators to rush the drying process for fear of frost. Dual path dryers, taking a longer time to dry each batch of corn, can force farmers to leave seed corn in the field longer. If exposed to sufficiently cold weather, the seed corn can lose viability, causing the value to the corn to drop from about $25 per bushel to about $2.50 per bushel. The choice to dry corn using single pass or dual path is thus optimally based on corn moisture, the predicted weather, as well as seed corn prices and energy prices.
The choice between single pass and dual path drying has heretofore been made at the time of dryer construction rather than the time of operation. Single pass corn dryers have not heretofore been switchable to operate as dual path corn dryers. Dual path corn dryers have not heretofore been switchable to operate as single pass corn dryers. What would be desirable and has not heretofore been provided is a seed corn dryer that can operate as either a single pass dryer or a dual path dryer.